Nucleation Undercooling Research Articles

Modeling of precipitate-free zone formed upon homogenization in a multi-component alloy

A comprehensive model is presented for the simulation of microstructure evolution during industrial solidification and homogenization processing of aluminum alloys. The model combines on the one hand microsegregation due to long-range diffusion during solidification and subsequent heat treatment with, on the other hand, precipitation in the primary Al phase. The thermodynamic data are directly obtained from a CALPHAD (CALculation of PHAse Diagrams) approach to thermodynamic equilibrium in multicomponent systems. The model is applied to the prediction of structure and segregation evolutions in a 3003 aluminum alloy for typical industrial solidification and homogenization sequences. It is shown that: (i) accounting for the nucleation undercooling of the eutectic/peritectic structures solidifying from the melt is essential to retrieval of the measured volume fractions of intergranular precipitates; (ii) calculations of intragranular precipitation are generally not applicable if long-range diffusion is neglected; (iii) the precipitate-free zone can be quantitatively predicted only based on the coupling between intergranular and intragranular precipitation calculations.

Analysis of microstructure formation in gas-atomised Al–12 wt.% Sn–1 wt.% Cu alloy powder

The microstructure of gas-atomised Al–12 wt.% Sn–1 wt.% Cu alloy powder has been investigated using scanning and transmission electron microscopy. Powder particles above approximately 8 μm in diameter exhibited a cellular–dendritic solidification morphology comprising α-Al dendrites and interdendritic Sn. In smaller powder particles, with diameters less than approximately 8 μm, the microstructure comprised an α-Al matrix with a dispersion of sub-micron Sn particles. The size of the dispersed Sn phase decreased with decreasing powder size (i.e. increased cooling rate). The formation of this dispersed phase microstructure is explained by the existence of a metastable liquid-phase miscibility gap in the system such that the reaction L → L 1 + L 2 occurred prior to the onset of solidification in particles below 8 μm in size. A heat transfer analysis was used to estimate the undercooling for the nucleation of α-Al in droplets cooled at different rates. The nucleation undercooling predicted for an 8 μm diameter droplet was 260 K and this represents a critical value which must be exceeded to enter the immiscibility region. The above value is in reasonable agreement with the temperature of the metastable immiscibility boundary, calculated from thermodynamic data, which occurs at an undercooling of approximately 280 K for Al–12 wt.% Sn.

Effect of thermosolutal convection on microstructure formation in the Pb-Bi peritectic system

Experimental studies have been conducted in the two-phase region of the Pb-Bi peritectic system to investigate the effect of thermosolutal convection on the banded microstructure. A systematic study is carried out by varying convection effects through the use of thin cylindrical samples of different diameters. A strong oscillatory convection is found in a 6.0-mm-diameter sample that produces a large treelike primary phase in the center of the sample that is surrounded by the peritectic phase matrix. The length of the treelike structure is found to decrease as the diameter of a sample is reduced to 0.8 mm. When the sample diameter is further reduced to 0.4 mm, laminar flow is present that gives rise to discrete bands of the two phases. The banded microstructure, however, is found to be transient and only the peritectic phase forms after a few bands. Composition variations in the banded structure are measured to determine the nucleation undercoolings for both phases and to characterize the banding cycle. The banding cycle is determined by the nucleation undercoolings and is independent of convection in the melt, but the banding window closely depends on convection. The presence of the transient banding process is analyzed by using a boundary layer model, and the number of transient bands is found to agree with the model for samples of different compositions and lengths.

Quantification of microsegregation during rapid solidification of Al-Cu powders

A new technique is introduced to quantify microsegregation during rapid solidification. The quantification involves calculation of the average solute solubility in the primary phase during solidification of an Al-Cu binary alloy. The calculation is based on using volume percent eutectic and weight percent of second phase (in the eutectic), which were obtained experimentally. Neutron diffraction experiments and stereology calculation on scanning electron microscope images were done on impulse atomized Al-Cu alloys of three compositions (nominal), 5 wt pct Cu, 10 wt pct Cu, and 17 wt pct Cu, atomized under N2 and He gas. Neutron diffraction experiments yielded weight percent CuAl2 data and stereology yielded volume percent eutectic data. These two data were first used to determine the weight percent eutectic. Using the weight percent eutectic and weight percent CuAl2 in mass and volume balance equations, the average solute solubility in the primary phase could be calculated. The experimental results of the amount of eutectic, tomography results from previous work, and results from the calculations suggest that the atomized droplets are in metastable state during the nucleation undercooling of the primary phase, and the effect of metastability propagates through to the eutectic formation stage. The metastable effect is more pronounced in alloys with higher solute composition.

Phase-field simulation of the columnar-to-equiaxed transition in alloy solidification

The columnar-to-equiaxed transition (CET) in directional solidification of alloys is simulated using the phase-field method. The method relies on the solution of a solute conservation equation and an equation for the propagation of the phase field on the scale of the developing microstructure. A parametric study is performed to investigate the effects of the applied temperature gradient and pulling speed, the seed spacing and nucleation undercooling for the equiaxed grains, and the crystalline anisotropy strength on the CET. The results qualitatively agree with a previously developed analytical model of the CET. At relatively high pulling speeds, a mixed columnar–equiaxed structure is found to be stable over a range of temperature gradients. Furthermore, the CET depends sensitively on the anisotropy strength. The simulations also reveal the presence of primary spacing adjustments during purely columnar growth due to nucleation of seeds, and deactivation of seeds by solutal interactions from nearby growing grains.

Effect of Catalyst on Heterogeneous Nucleation in Fe-Ni-Cr Alloys

In order to elucidate the nature of heterogeneous nucleation, a differential scanning calorimetry (DSC) thermal analysis of Fe–Ni–Cr alloys (Ni+Cr content=30 mass% (constant); Ni content: 7.5 to 29.3 mass%) containing TiN and Al2O3 was conducted. Then, special attention was paid to the difference in the phase of the primary crystal nucleated by the triggering effect of a catalyst (nucleating agent). The solidification and transformation mode appearing during cooling in these alloys is classified into three cases: F mode, FA mode, and A mode. The change of modes and the critical undercooling (ΔT) depend on the kind of catalyst used as well as on the chemical composition. In addition, in spite of the kind of primary crystal, the value of ΔT is always smaller in the order of TiN<Al2O3. These findings are the same as those for Fe–Ni alloy, although Fe–Ni–Cr alloys have the advantage that the increase of Cr content can trigger the crystallization of the primary crystal of the δ phase, which consequently can reduce ΔT and extend the region of low ΔT. The total concentration of catalyst molecules (C1), as well as the interface free-energy ratio ((γCL−γSC)/γLS), is one of the most significant factors in explaining undercooling for heterogeneous nucleation. (γCL−γSC)/γLS is related to the kind of catalyst used and the chemical composition, and C1 is related to the crystallized content of catalyst. As a matter of fact, the number of catalysts supplied to the solidification front has a practical effect on the control of the solidification-structure.

Transient eutectic solidification in In–Bi–Sn: Two-dimensional experiments and numerical simulation

A technique widely used for the in situ observation of microstructure formation in transparent organic systems was applied to metallic alloys with low melting temperature. In the ternary eutectic alloy system In–Bi–Sn, we observed by light microscopy steady state and transient eutectic microstructure formation during unidirectional solidification in thin (5–10 μm) glass capillaries. Starting from an equilibrated planar single-phase front in a slightly off-eutectic alloy we studied the dynamic processes like nucleation of univariant, and finally invariant ternary eutectic when the liquid transforms into three distinct solid phases. Numerical microstructure simulation was performed by applying a general multiphase and multi-component phase field model which couples directly to the complete thermodynamic dataset of the In–Bi–Sn alloy system. Via comparison of the experimental to the simulated data some thermo-physical values like diffusion coefficients and critical undercooling for nucleation have been evaluated.

Influence of grain refinement on the columnar-to-equiaxed transition in binary Al alloys

In the solidification of metallic alloys, columnar or equiaxed growth may occur depending on, among others, the experimental conditions, the thermophysical properties of the alloy and the presence of grain refining particles. In transient processes, typical for many casting situations, a columnar-to-equiaxed transition (CET) is often observed. We have investigated the CET experimentally in AlSi7 alloys with and without inoculants within the framework of the ESA microgravity application promotion programme (MAP) columnar-to-equiaxed transition in solidification processing (CETSOL). The effect of different cooling rates on the CET was studied in addition to the effect of grain refinement in directional solidification. From measured temperature profiles in the samples during solidification and from the analysis of longitudinal and transversal sections of the samples, critical parameters at the CET were determined and compared to predictions from numerical models following Martorano et al. and from Hunt. It was found that the results are in good agreement with the former for non-refined AlSi7 alloys and that grain refinement shifts the critical parameters significantly, concerning critical gradient and nucleation undercooling.

Phase field modelling of the interfacial condition at the moving interphase during the γ → α transformation in C–Mn steels

The phase field model is used to simulate the γ → α transformation in an Nb microalloyed C–Mn steel during cooling from different austenitisation temperatures. The model provides qualitative information on the developing microstructure, as well as quantitative information on both the ferrite fraction formed and the carbon concentration distribution in the remaining austenite. The initial austenitic microstructure (grain morphology and dimensions) and the nucleation conditions (nucleus density and nucleation undercooling) are set as input data of the model; they are derived by metallographic and dilatometric tests. The interface mobility is used as a fitting parameter to optimise the agreement between the experimental ferrite fraction curve, obtained by dilatometry, and the simulated ferrite fraction. A strong decrease of the γ/α interface mobility is found when the austenitisation temperature decreases from 1373 to 1173 K. A good agreement is obtained between the simulated microstructure and the actual microstructure observed after cooling.

THERMAL ANALYSIS STUDY OF THE EFFECT OF THE COOLING RATE ON THE MICROSTRUCTURE AND SOLIDIFICATION PARAMETERS OF 319 ALUMINUM ALLOY

In the metal casting industry, an improvement of component quality depends mainly on better control over the production parameters. Thus, a thermal analysis cooling curve of the alloy is used for process control in the aluminum casting industry. In this research, the effect of seven different cooling rates on the microstructure and solidification parameters of the 319 aluminum alloy have been investigated by means of thermal analysis. In each case, the cooling curve and its first derivative curve have been plotted by using very accurate thermal analysis equipment. The effect of different cooling rates on solidification parameters such as nucleation temperature (TN,α), nucleation undercooling (∆TN,α), recalescence temperature (TR,α), solidification range (∆Ts) and total solidification time (tf) has been studied in the liquidus region. Microstructural evaluation has been carried out and DAS was measured in all solidification conditions.Dans l’industrie de coulage des métaux, une amélioration de la qualité d’une composante dépend principalement d’un meilleur contrôle des paramètres de production. Ainsi, une analyse thermique de la courbe de refroidissement de l’alliage est utilisé pour le contrôle de procédé dans l’industrie de coulage de l’aluminium. Dans cette recherche, on a utilisé l’analyse thermique pour étudier l’effet de sept différents taux de refroidissement sur la microstructure et sur les paramètres de solidification de l’alliage d’aluminium 319. Dans chaque cas, on a tracé la courbe de refroidissement ainsi que la courbe de sa première dérivée en utilisant de l’équipement très précis d’analyse thermique. On a étudié, dans la région du liquidus, l’effet de différents taux de refroidissement sur les paramètres de solidification comme la température de nucléation (TN,α), la surfusion de nucléation (∆TN,α), la température de recalescence (TR,α) l’étendue de la solidification (∆Ts) et la durée totale de solidification (tf). On a effectué une évaluation de la microstructure et l’on a mesuré DAS sous toutes les conditions de solidification.

Effect of Twin Growth on Unidirectional Solidification Control of Multicrystal Silicon for Solar Cells

The solidification microstructure and crystal orientation have been investigated for solar cell grade high purity multicrystal silicon through a unidirectional solidification technique. The mechanism of the twin growth on a reentrant corner has been also discussed. A columnar structure is observed at solidification velocities of 1.25–30 μm/s and positive temperature gradient of 20 K/cm in the rod-like silicon specimens in an electric resistance furnace. In the solidification velocity range of 1.25–2.5 μm/s, the grain size enlarges as solidification progresses. Furthermore, large columnar grains contain many twin boundaries. However, the average grain size decreases as the solidification velocity increases and above the critical velocity around 40 μm/s, equiaxed structure appears at the central part of specimens. Therefore, molten silicon must be solidified at the velocity below 2.5 μm/s where twins are always introduced into grains to obtain large columnar crystal grains. The undercooling for directional growth is less than 4 K in the solidification velocity range of 1.25–30 μm/s. A model of two-dimensional nucleation on the reentrant corner was established, and the critical nucleus could be estimated to be 70 to 80% of the radius of the general two-dimensional nucleus. The nucleation undercooling on the surface containing twins also decreased to 70% of the general undercooling. The reduction of the critical radius and undercooling on the reentrant corner could eventually influence on the priority growth direction and the enlargement of the grain size.

Nucleation and Growth in Undercooled Melts of Bulk-Metallic-Glass Forming Zr&lt;SUB&gt;60&lt;/SUB&gt;Ni&lt;SUB&gt;25&lt;/SUB&gt;Al&lt;SUB&gt;15&lt;/SUB&gt; Alloy

A levitation method using alternating and static magnetic fields was used to measure nucleation and growth of the crystalline phases in melts of the bulk-metallic-glass forming Zr 60 Ni 25 Al 15 alloy. For comparison, Zr 66.7 Ni 33.3 and Zr 66.5 Ni 33.2 Al 0.3 were also examined. Nucleation undercooling in the bulk-metallic-glass forming Zr 60 Ni 25 Al 15 alloy did not depend on the cooling rate (<10 2 K/s), and the maximum undercooling observed was approximately 200K. Recalescence was not observed in the cooling curves of the Zr 60o Ni 25 Al 15 and the Zr 66.5 Ni 33.2 Al 0.3 alloys. The growth velocity of the crystalline phase in the Zr 60 Ni 25 Al 15 alloy was of the order of 10 -4 m/s even at a undercooling of 100 K. In contrast, clear recalescence was always observed in the Zr 66.7 Ni 33.3 alloy. The results showed that the addition of Al into the Zr-Ni alloy significantly reduced growth velocity of the crystalline phases. The extremely low growth rate in the Zr-Ni-Al system can contribute to the high glass forming ability.

A novel experiment for the study of substrate-induced nucleation in metallic alloys: Application to Zn-Al

In order to investigate the nucleation and growth mechanisms of Zn-0.2 wt pct Al grains in thin coatings deposited on steel sheets by hot dipping, a new directional solidification experiment has been specifically designed with a steel substrate immersed into the melt. The competition (CTG) between regular columnar (C) grains growing parallel to the thermal gradient and transverse grains (TGs) nucleated on the sheet surface and growing transversely has been observed in longitudinal sections of Zn-0.2 wt pct Al (-Sb) alloys. Without Sb, the CTG began at midheight of the casting, whereas with Sb, no CTG was found. A simple CTG criterion similar to that of Hunt for the columnar-to-equiaxed Transition (CET) was derived. The calculated extension of the TGs was successfully compared with that measured on micrographs reconstructed from electron backscattered diffraction (EBSD) measurements. This indicated that the critical nucleation undercooling for the formation of Zn-0.2 wt pct Al grains on the steel sheet is about 0.7 K and that Sb poisons these available nucleation sites.

Phase and microstructure selection in peritectic alloys under high G-V ratio

Abstract A new model for the case of a superposition of a phase transformation and a microstructure transition from columnar plane front to equiaxed dendritic growth for peritectic alloys is presented. This model is applied to Fe–Ni alloys where a phase transition from γ to δ takes place when the solute content decreases. The results of the model are compared with predictions of the nucleation constitutional undercooling model. They show that the peritectic two-(solid) phase region is restricted and may completely disappear depending on nucleation undercooling and dendrite growth kinetics.

Effect of droplet size on nucleation undercooling of molten metals

Effect of droplet size on nucleation undercooling of molten metals

Critical undercoolings for the transition from the lateral to continuous growth in undercooled silicon and germanium

The effects of crystal–liquid interface energy and undercooling on a crystal–liquid interface structure and the growth mode of undercooled semiconductors have been investigated. A factor to predict the structure of the solid–liquid interface has been identified. Criterions expressed as the critical nucleation undercoolings (Δ T * 1 and Δ T * 2) and kinetic undercoolings (Δ T K1 * and Δ T K2 *) to judge the transition of growth mode in undercooled semiconductor have been developed. The growth mode of a semiconductor is a lateral one when nucleation undercooling is lower than Δ T * 1 or kinetic undercooling is lower than Δ T K1 *, a continuous one when nucleation undercooling is higher than Δ T * 2 or kinetic undercooling is higher than Δ T K2 *, and an intermediary one in the nucleation undercooling region from Δ T * 1 to Δ T * 2 or kinetic undercooling region from Δ T K1 * to Δ T K2 *. The critical undercoolings for the growth transition predicted from the present criterions for silicon and germanium are in very good agreement with the experimentally measured results.

Growth Analysis of Banded Microstructure Formed during Directional Solidification of Peritectic Alloys

In this work, a growth analysis is made on the banded microstructure formation during directional solidification of peritectic alloys. The convection model, which has been proposed to explain the discrete banded microstructure formation, is extended in the part of the nucleation condition. Applying the nucleation and constitutional undercooling criterion, a new calculation was developed. Concentration profile calculated by the model was compared with experimentally obtained banded microstructure in Fe-Co and also with those of the convection and the diffusion models. Consistent agreement with experiments is shown. In comparison with the other models, interesting difference in predictions of concentration profile is obtained between them.

X-RAY TOMOGRAPHY STUDY OF ATOMIZED Al-Cu DROPLETS

The atomization of metals and alloys results in the rapid solidification of droplets which yields a complex interplay of nucleation undercooling, recalescence and microsegregation. Characterization of the solidified microstructure has been studied using conventional optical and scanning electron microscopy (SEM). These techniques provide limited information as only a finite number of sections of the powder can be examined. X-ray tomography using synchrotron radiation with a 1 μm beam resolution was used to generate a 3D image for the entire volume of a particle and was compared to microscopic images. The tomography experiments were carried out on Al5%Cu and Al17%Cu atomized droplets of 500 μm in diameter. Microstructural features of solidification such as shrinkage, nucleation and recalescence which are apparent in the 3D images are discussed and compared with conventional characterization methods.L’atomisation de métaux et d’alliages résulte en une solidification rapide de gouttelettes qui produit une interaction complexe de la surfusion de nucléation, de la recalescence, de la microségrégation, etc. On a caractérisé la microstructure solidifiée en utilisant la microscopie conventionnelle optique et électronique à balayage. Ces techniques fournissent une information limitée puisque l’on peut examiner seulement un nombre défini de sections de la poudre. On a utilisé la tomographie à rayonnement cyclotron avec résolution du faisceau de 1 μm pour générer une image à 3D du volume entier d’une particule que l’on a comparée aux images obtenues par microscopie. On a effectué les expériences de tomographie avec des gouttelettes atomisées d’Al-5%Cu et d’Al- 17%Cu, ayant 500 μm de diamètre. On discute des caractéristiques de la microstructure de solidification comme la contraction, la nucléation, la recalescence, etc. qui sont apparentes dans les images à 3D et on les compare avec les méthodes conventionnelles de caractérisation.

A solutal interaction mechanism for the columnar-to-equiaxed transition in alloy solidification

A multiphase/multiscale model is used to predict the columnar-to-equiaxed transition (CET) during solidification of binary alloys. The model consists of averaged energy and species conservation equations, coupled with nucleation and growth laws for dendritic structures. A new mechanism for the CET is proposed based on solutal interactions between the equiaxed grains and the advancing columnar front—as opposed to the commonly used mechanical blocking criterion. The resulting differences in the CET prediction are demonstrated for cases where a steady state can be assumed, and a revised isotherm velocity (VT) vs temperature gradient (G) map for the CET is presented. The model is validated by predicting the CET in previously performed unsteady, unidirectional solidification experiments involving Al-Si alloys of three different compositions. Good agreement is obtained between measured and predicted cooling curves. A parametric study is performed to investigate the dependence of the CET position on the nucleation undercooling and the density of nuclei in the equiaxed zone. Nucleation undercoolings are determined that provide the best agreement between measured and calculated CET positions. It is found that for all three alloy compositions, the nucleation undercoolings are very close to the maximum columnar dendrite tip undercoolings, indicating that the origin of the equiaxed grains may not be heterogeneous nucleation, but rather a breakdown or fragmentation of the columnar dendrites.

Preparation and characterization of SiO 2 non-catalytic nucleation coating for the solidification of undercooled melt*

Abstract SiO2 non-catalytic nucleation films were coated onto a ceramic substrate and the inner surface of a ceramic crucible by the Sol-Gel method. The surface morphology of the coatings was characterized with an atomic force microscope and a scanning electronic microscope. The non-catalytic effect of the coating on the heterogeneous nucleation of metal melts was tested through measuring the nucleation undercooling. Investigation results indicated that the prepared coating could act as a baffle-wall to prevent the metal melt from nucleating heterogeneously on the inner surface of the crystalline crucible. The tiny holes with a diameter below 2 μm and heaves on the coating surface have no evident effect on the undercooling of metal melts. For a poly-layer coating the super-layercan cover the cracks in the sublayers which prevents the cracks in the coating from spreading in the direction vertical to the coating layers and thus eliminates the effect of cracks on the coating.